George, being the friend that he was, did not want Lennie to suffer a great deal, and he knew that if Curly got a hold of Lennie he would be suffering a great deal.
George wanted to prevent his friend from suffering so much, and so he decided that it would be better if he shot his own friend.
Candy shows that companionship and hope are necessary to survive.
His best friend and lifelong companion were his sheep dog.
The friendship Lennie and George had in Of Mice and Men is incomparable to most friendships today.
They were two very different people brought together by fate.
Through their many differences these two prove that they need each other, and through each other they learn how to survive. Lennie helps George through his brute strength, while George helps Lennie control his strength. He thinks of ways to earn their money, and he helps control Lennie and make sure that he behaves himself.
The two different characters manage not to clash, but instead balance each other out.
Through all the hardships that they've been through, they still managed to pull through it all, that is, until it came to the very end. Lennie a muscular guy with no sense of what is going on around him, relies purely on what George tells him to do.
Lennie is all brute strength without a hint of intelligence in his body.